For the time being I'm still using veroboard for my projects. I use my dad's model railway track cleaner thing that looks like an eraser and is very slightly abrasive to clean the veroboard tracks before soldering. I find it makes the solder flow really well but the problem is that when i've finished and take a look at the copper tracks a week later they've all gone really dull, like they've corroded, as if by cleaning it i've removed some kind of anti-corrosion coating.

So, is it best to cut the board to size, clean it and then give all the copper tracks a thin coating of solder? What else can I do?

There are tin plating solutions available, I believe GC used to sell one, all you do is dump your circuit board into the solution and remove it and rinse a few minutes later, your copper is now silver colored as it now has a thin tin plated surface.